Consider a digital oscilloscope with at least two vertical input channels. Since the display is created by raster scan techniques from sampled data stored in a memory, a color CRT or flat panel display can produce a different color for the trace corresponding to each channel. Once this level of functionality is available, it is easy to allow the user to select which color is to be associated with each channel. The justification for such flexibility may be mere preference on the one hand, or it may approach necessity on the other; say, either because of a particular kind of color blindness on the part of the operator or some unusual ambient lighting conditions.
On the other hand, such flexibility raises the issue of whether or not to indicate on the front panel which set of channel controls is associated with each differently colored trace on the screen. That is, suppose that the blue trace has trace excursions that go off-screen, indicating a need to decrease the vertical sensitivity for that channel. Because the correspondence between color and channel is user selectable, it is essentially arbitrary. Absent some indication from the instrument, it is not always possible to quickly and easily decide which is the correct vertical sensitivity control to adjust. Conventional permanently color coded lettering on the front panel is appropriate when the correspondence between channel and trace color is fixed, but not when it is arbitrary.
The ease of use of a sophisticated instrument having selectably different colors for aspects of its operation, and controlled by respective groups of front panel controls, can be enhanced by eliminating the confusion about which group of controls goes with what color results in the display.